Beta 1 of WordPress 2.6 has been released! If you want to test it safely, you should be able to do so as soon as the WordPress 2.6 demo site is updated.
Update: Robert posted a comment pointing out an alternate demo site.
Although WordPress 2.6 is still in the early alpha stages (as of this writing), you can still check out the unofficial WordPress 2.6 Demo Site and give some of the new features a whirl.
Weblog Tools Collection is running a WordPress Plugin Competition! I have several plugins I’m working on that I’m planning to submit. Stay tuned for more info on their releases.